A MEEN stack developer works on developing web applications using a specific set of technologies: MongoDB, Express.js, Ember.js, and Node.js. They design and implement the data storage using MongoDB, a NoSQL database. Express.js, a server-side framework, and Node.js, a JavaScript runtime, are used for managing server and backend aspects. Ember.js, a client-side JavaScript web application framework, is used for frontend development. They are responsible for writing reusable, testable, and efficient code. They also manage user authentication and authorization between multiple systems, servers, and environments. Furthermore, they integrate data from various back-end services and databases.